Semiconductor chips are comprised of a series of integrated circuits and devices which include transistors. Transistors are of two general types, namely bipolar and field effect transistors (alternately termed FET). Bipolar transistors are comprised of regions and junctions of p-type and n-type material, and are comprised of either "npn" or "pnp" configurations. The three regions are designated as an emitter, a base and a collector. One common type of FET transistors are MOS transistors. These are also comprised of three regions termed a drain, a gate and a source. In operation of a MOS transistor, a voltage applied to the gate region causes a conductive channel to form between the source and drain within the substrate, resulting in current flow.
Complementary MOS (CMOS) is a MOS circuit formed with both n-channel and p-channel devices, namely an n-type transistor (nMOS) and a p-type transistor (pMOS). The nMOS transistors have the source and drain regions formed of n-type material in a p-type substrate or p-well, while pMOS transistors have source and drains formed of a p-type material in an n-type substrate or n-well. CMOS allows more powerful circuit operation than either n-channel or p-channel circuits alone. This factor, combined with the lower power consumption and increased speed, has made CMOS the favored technology for the manufacture of microprocessors and memory devices.
In certain situations, it is desirable to form bipolar transistors within substrate wells containing CMOS transistors. Such processes are commonly known as BiCMOS processes.
In wafer fabrication, circuit formation in and on the wafer requires many sophisticated processes. The actual number of steps the wafer goes through will vary with the type and complexity of the circuit being built. CMOS and BiCMOS processes tend to include large numbers of individual processing steps, which include several masking steps. Despite the many steps, only three basic operations are performed on a wafer:
layering--thin layers of different materials are grown on, or added to the wafer surface. PA1 patterning--portions of thin layers are defined and selectively removed or etched from the wafer. PA1 doping--the resistivity and conductivity type of selected regions in the wafer are changed by the addition of dopants.
It would be desirable to incorporate bipolar devices into a CMOS process (i.e., BiCMOS) without adding additional masking steps. It would also be desirable to devise processes for formation of bipolar transistors in semiconductor wafers which minimize the number of masking steps.